(1) Field of the Invention
The present invention relates to a static magnetic filed applying structure for use in an atomic oscillator. In particular, the present invention relates to a structure for applying a static magnetic filed for use in an atomic oscillator of a passive-type based on a principle of optical pumping.
(2) Description of Related Art
Recently, the information communication network tends to move forward to a digital network construction, and with this tendency, it becomes indispensable to prepare a clock source which can provide a highly accurate and highly stable clock. A rubidium atom oscillator is drawing attention as the clock source, and it is requested to establish a way of small-sizing and low-cost production. In particular, the recent request is to establish a way of fabricating the oscillator having a thin body from a device-mounting standpoint.
How the atomic oscillator is made into a thin body depends on how an optical-microwave resonator can be miniaturized, and many manufacturing companies pay a lot of effort for devising a construction which makes it possible to small-sizing the microwave oscillator. Recent released report introduces a new method of study achievement. That is, the microwave resonator is obviated but a laser-light beam is subjected to a microwave modulation and this laser-light beam is effected to cause an atomic resonance (dark line resonance method). Thus various approaches have been made to establish a device small-sizing.
These methods stand on the same fundamental principle. That is, the atomic oscillator employs transition between energy levels in atomic ultra-micro structure. Therefore, it is necessary to keep an atom in a static magnetic field and cause Zeeman effect to divide the energy level. To this end, it is indispensable to provide a circuit for generating a static magnetic field. This is a critical development item for proceeding the small-sizing and the low-cost production in the later stage.
FIG. 10 is a diagram for explaining a function and a structure of a conventional rubidium atomic oscillator. In FIG. 10, reference numeral 101 represents a first magnetism shielding structure, 102 a second magnetism shielding structure, and 103 and 104 thermal insulating materials.
Further, reference numeral 105 represents a resonance cell having rubidium atoms enclosed therein and absorbing a ray of light having a particular wavelength by using atomic energy level transition caused on the rubidium atoms. Reference numeral 106 represents an optical detector for detecting a ray of light passing through the resonance cell 105, 107 a cavity (resonance cavity) for accommodating the resonance cell 105, 108 a varactor diode for gradually multiplying a degree of microwave, 109 a solenoid coil capable of generating a magnetic field for adjusting a resonance frequency of the rubidium atoms enclosed in the resonance cell 105, 110 a rubidium lamp for emitting resonance light, 111 a lamp housing for accommodating the rubidium lamp, 112 a lamp excitation circuit for exciting the rubidium lamp 110 by energizing the rubidium lamp 110 with high frequency microwave, respectively. The aforesaid resonance cell 105, the optical detector 106, the cavity 107, the varactor diode 108, a solenoid coil 109, the rubidium lamp 110, the lamp house 111 and the lamp excitation circuit 112 constitute an OMU (Optical Microwave Unit).
Further, reference numeral 113 represents a heater for keeping the temperature of the OMU constant, and 114 a thermistor capable of varying its resistance depending on the temperature change of the OMU. In FIG. 10, a portion including components of from the first shielding structure 101 to the thermistor 114 schematically shows a cross-section of a double-layer housing structure made of the magnetism shielding structures 101 and 102 accommodating the resonance cell 105 or the like of the components constituting the rubidium atom oscillator.
Furthermore, reference numeral 115 represents a temperature control unit for detecting the resistance of the thermistor 114 and generating a voltage for controlling the temperature of the aforesaid heater 113, 116 a transistor for controlling an electric current to be supplied to the heater depending on the output of the temperature control unit 115, 117 a preamplifier for amplifying the output of the optical detector 106, 118 a low-frequency oscillator for generating a low frequency signal, 119 a synchronism detecting circuit for detecting synchronism in the output of the preamplifier 117 in response to the output of the low frequency oscillator 118, 120 a frequency control circuit for controlling an oscillation frequency of a voltage-controlled crystal oscillator, which will be described later on, depending on the output of the synchronism detecting circuit 119, 121 the voltage-controlled crystal oscillator of which oscillation frequency can be stabilized by using the atomic resonance in the resonance cell 105, and 122 a frequency modulating circuit for modulating in phase the output of the voltage-controlled oscillator 121 in response to the output of the low-frequency oscillator 118 and supplying the same to the aforesaid varactor diode 108, respectively.
Now the principle on which the operation of the rubidium atomic oscillator is based will be described with reference to FIG. 11.
As shown in FIG. 11A, if the rubidium atoms hermetically enclosed within the resonance cell 105 shown in FIG. 10 are placed in a thermal equilibrium state, the rubidium atoms reside in the (5S, F1) level which is a ground state and the (5S, F2) level at an equal probability. Under this state, if a resonance ray of light of the rubidium lamp 110 is irradiated onto the resonance cell 105, then as shown in FIG. 11B, only the rubidium atoms reside in the (5S, F1) level are excited and pumped up to the 5P level. This phenomenon is known as an optical pumping (excitation). However, since the 5P level is an unstable energy level, as shown in FIG. 11C, owing to a spontaneous emission, the atoms residing in this level transit to the (5S, F1) level which is a ground state and the (5S, F2) level at an equal probability.
Thereafter, the resonance ray of light of the rubidium lamp 110 causes the optical pumping leading to excitation that only the rubidium atoms residing in the (5S, F1) level are excited to the 5P level, and the spontaneous emission causes the transition from the 5p level to the (5S, F2) level at equal probability. In this way, this process of excitation and spontaneous emission is repeated. Owing to these repeating processes, as shown in FIG. 1D, a state is brought about in which almost all rubidium atoms populate in exclusively the (5S, F2) level. This state is known as a negative temperature state. Under this state, if a microwave deriving from the output of the frequency modulating circuit 122 undergoing the gradual multiplication process in the varactor diode 108 is effected to the cavity 107 to excite the same, as shown in FIG. 11E, transition is caused on the rubidium atoms residing in the (5S, F2) level to the (5S, F1) due to an induced emission.
At this time, the resonance cell 105 absorbs the optical energy generated from the rubidium lamp 110, with the result that the optical level detectable by the optical detector 106 will be lowered. Then, the probability at which the rubidium atoms residing in the (5S, F2) level transit to the (5S, F1) level will be maximized when the frequency of the microwave is coincident with a frequency corresponding to the energy difference between the (5S, F2) level and the (5S, F1) level (this frequency is known as resonance frequency). Conversely, the probability will be lowered as the difference between the frequency of the microwave and the resonance frequency.
That is, as shown in FIG. 12A, the output of the optical detector 106 increases as the distance of the microwave frequency with respect to the resonance frequency f0 which corresponds to the difference between the (5S, F2) level and the (5S, F1) level. Finally, since the induced emission owing to the microwave is not caused, the optical detector output becomes constant. The concave portion near the resonance frequency f0 of a curve A is known as “dip”.
Meanwhile, since the output of the voltage controlled crystal oscillator 121 is subjected to a phase modulated by the low-frequency oscillator 118, frequency variation is caused in a frequency at which the varactor diode 108 excites the cavity. For this reason, the light absorption efficiency of the resonance cell 105 is changed and the optical level detected by the optical detector 106 is also changed. Initially, if the excitation frequency f of the microwave by the varactor diode 108 is equal to the resonance frequency f0 of the resonance cell 105, the excitation frequency f of the microwave deriving from the varactor diode 108 modulated with the low-frequency signal is changed near the bottom portion of the aforesaid dip. Therefore, as shown at reference B in FIG. 12A, the output of the optical detector 106 becomes a signal having a frequency twice the low-frequency modulated frequency.
On the other hand, if the excitation frequency f of the microwave by the varactor diode 108 is higher than the resonance frequency f0 of the resonance cell 105, the excitation frequency f of the microwave deriving from the varactor diode 108 modulated with the low-frequency signal is changed at the rising-up part of the right side. Therefore, as shown at reference C in FIG. 12A, the output of the optical detector 106 changes its phase relative to the low-frequency signal. Conversely, if the excitation frequency f of the microwave by the varactor diode 108 is lower than the resonance frequency f0, the excitation frequency f of the microwave deriving from the varactor diode 108 modulated with the low-frequency signal is changed at the rising-up part of the left side. Therefore, as shown at reference D in FIG. 12A, the output of the optical detector 106 changes its phase opposite to the low-frequency signal.
The output of the optical detector 106 varying as described above is led through the preamplifier 117 to the synchronism detecting circuit 119 in which synchronism is detected with the output of the low-frequency oscillator 118. That is, the output of the optical detector 106 amplified by the preamplifier 117 is supplied to the frequency control circuit 120 in which proportional control, integration control, differentiation control and a combination of these control schemes is effected to the supplied signal to generate a control voltage (see FIG. 12B) which is to be supplied to the voltage-control crystal oscillator 121. With this control voltage, the voltage control crystal oscillator 121 is controlled so that the output thereof becomes equal to the resonance frequency f0 of the resonance cell 105. Thus the output of the rubidium atomic oscillator is supplied to the outside.
As described above, the rubidium atomic oscillator employs the principle of transition between energy levels of atoms. However, the energy level utilized in the oscillator takes a ultra-micro structure in which each of the levels is separated from others by Zeeman effect. Therefore, it is necessary for the rubidium atoms to be kept in a static magnetic field. In a conventional product, this condition is realized by involving the resonance cell 105 within the solenoid coil 109. The static magnetic field which can be created by the solenoid coil 109 employed by the conventional product is about H≈45 (A/m).
Description will be hereinafter made on a construction of a conventional static magnetic field (generating) circuit. FIG. 13 is a diagram schematically showing the construction thereof. As shown in FIG. 13, the static magnetic field generating circuit comprises, for example, a cylindrical cavity resonator 107 having a light beam passing aperture through which a pumping light beam can be incident into the resonator, a toroidal dielectric body 123 to be involved in the cavity resonator 107 so at to attain small-sizing of the cavity resonator 107, an antenna 124 for exciting a microwave within the cavity resonator 107, a gas cell (resonator cell) 105 having rubidium atoms enclosed therein, an optical detector 106 for detecting a light beam passing through the gas cell 105, an adjusting screw 125 for adjusting the resonating frequency of the cavity resonator 107 to the resonance frequency of the rubidium atoms, and a solenoid coil 109 for retaining the rubidium atoms in the static magnetic field. In FIG. 13, the cavity resonator 107 is schematically shown in a cross-sectional manner.
As is disclosed in FIG. 5 of the following patent document 1, the solenoid coil 109 is fabricated by winding a copper wire around a metal wall forming the cavity resonator 107. Although illustrated is one having a winding wire directly wound around the metal wall, the winding portion may be prepared in advance in a manner that a wire is wound around a cylindrical member made of a resin or the like (mechanical part) and this mechanical part having the wire wound therearound may be attached to the cavity resonator 107. Further, one of conventional technologies employs a cavity resonator having not a cylindrical shape but a rectangular piped shape. Also in this case, similarly to the case of the cylindrical shape, the copper wire or the like may be directly wound around the metal wall of the cavity resonator 107 to form the solenoid coil. Alternatively, the wire may be wound around a case made of resin or the like having a thermally insulating function to form the solenoid coil 109 and this solenoid coil may be attached to the cavity resonator 107.
Further, as is disclosed in FIGS. 1 and 6 of the following patent document 2, a wire may be wound around a bottom portion of a yoke having a U-letter cross-section and an electric current may be supplied to the wire so that a magnetic flux may be created from one end of the yoke to the other of the same which are opposing to each other. Thus, a static magnetic field (C field) can be applied to the resonator of a cesium atom oscillator.
[Patent Document 1] Japanese Patent Application Laid-Open No. HEI 6-334520
[Patent Document 2] Japanese Utility Model Application No. SHO 61-100049
Recent request from the market is to attain small-sizing and low-cost production of the rubidium atom oscillator. To this end, many manufacturing companies try to establish technologies enabling the peripheral circuits to be small-sized. However, further small-sizing and low-cost production are requested. In order to respond to the requests, it is indispensable to achieve small-sizing and low-cost production on the OMU. Conventional approaches for this purpose have been made based on a scheme that the mode of the cavity resonator was selectively activated depending on the use mode thereof, the cavity resonator was filled with a material having a high relative dielectric constant and this cavity resonator was provided in the rubidium atom resonator. Similar approaches have been tried and various approaches will be made for attaining the small-sizing of the oscillator from now on. A problem we will encounter at this time is how the static magnetic field generating circuit shall be handled. In the conventional structure described with reference to FIGS. 10 and 13, it is necessary to dispose the resonator cell 105 within the solenoid coil 109. Therefore, layered structure is requested for the static magnetic field generating circuit, with the result that the structure thereof necessarily becomes complicated and freedom of structure arrangement is limited. This fact becomes a problem to be solved when further small-sizing and low-cost production are carried forward.
In particular, when the aforesaid small-sizing and the low-cost production are intended on a thin-shaped products which are recently demanded from the market, a cavity resonator for use in microwave excitation, a semi-coaxial resonator, a dielectric resonator or the like will have a rectangular shape because the aforesaid thin-shaped design is more easily achieved. However, the resonator cell shall be provided within the static magnetic field and hence the solenoid coil shall be formed so as to surround the resonator. Therefore, to increase the thickness of the wire winding portion is unavoidable. Further, the resonator requires a frequency adjusting mechanism. For this reason, the solenoid coil frequently encounters limitation in the winding allowance portion around which the wire is wound. As a consequence, parts constituting the solenoid coil are complicated and high-cost. Some makers employ a scheme of winding a wire directly around the resonance cell to form a coil and a static magnetic field is applied thereto. However, a process for winding a wire to form a solenoid coil is essentially indispensable and hence custom-arrangement is requested depending on mechanical parts combination for handling each case of design. Accordingly, cost increase is unavoidable.
Meanwhile, according to the technology disclosed in the aforesaid patent document 2, the structure has a yoke with a U-letter shape in cross-section provided and a magnetic flux is generated from one end to the other end of the side portions opposing to each other so that the resonance device of the cesium atom oscillator is applied with a static magnetic field. Therefore, the resonance device need not be surrounded with a solenoid coil as described above. Therefore, it becomes possible to enhance a thin-shaped design as compared with a case in which the resonance device is surrounded with a solenoid coil. However, the structure in this case requires certain areas corresponding to the size of the resonance device at the bottom portion and the side portion of the U-letter shaped yoke, respectively. Therefore, the mounting area reduction is also limited. Moreover, a process for winding the wire at the bottom portion of the yoke is indispensable and hence custom-arrangement is requested depending on mechanical parts combination for handling each case of design. Accordingly, cost increase is unavoidable.